1. Field of the Invention
The present invention relates to a preparation of a look-up table for converting an input color signal into a plurality of color components including a black component.
2. Related Background Art
In order to form a device dependent C (cyan) M (magenta) Y (yellow) K (black) output value from an input R (red) G (green) B (blue) value from an application, it is known that a color printing apparatus is provided with means for converting the input RGB signal value into a device dependent RGB value and a color conversion LUT for converting the device dependent RGB value into the CMYK output value.
The decision of a K ink amount and an under color removal amount in each lattice point of the color conversion LUT have been calculated from a function with the minimum value of each CMY or K ink amount taken as a parameter. It is difficult to prepare a function for adequately controlling the K ink amount and, depending on a under color removal function, a graininess may have been often produced in an output of high lightness with a result that the image quality of an output image has been deteriorated.
Further, in the under color removal function and a method for preparing the color conversion LUT by a masking calculation method and also a method for performing a three dimensional interpolation in a L*a*b* color space from a three dimensional printer model and for searching a desired CMYK ink amount, not only a vast amount of calculation times have been required, but also an irregularity of gradation has been produced in an output result. Furthermore, with these methods, it cannot always be said that an output color space in the color printing apparatus has been reproduced up to a maximum and that sufficient color reproduction has been maintained for the RGB signal color space which is the input signal.
When the maximum density of the K ink itself is thin, in case of placing the maximum output value of the K ink only on a process black, an output density by a combination of three ink systems to be complementary colors for the adjacent chromatic color color ink, the K ink and the chromatic color color ink is higher than the process ink, and this often gives a rise to the problem and the like that a distortion is produced in the output color space for the color printing apparatus and that a quasi contour is produced in the output image.